Certain types of refrigeration units, such as transport refrigeration units suitable for conditioning cargo spaces of trucks and trailers, often have a prime mover for the refrigerant compressor which includes an internal combustion engine. Such refrigeration units are often operable in a selected one of two different operating modes, as selected by an operating mode switch. When the operating mode switch is in a first position, the prime mover is operated continuously, with the refrigeration unit cycling between cooling and heating cycles once a selected set point temperature in an associated conditioned space has been reached.
When the operating mode switch is in a second position, the prime mover is operated in a start-stop cycle mode, in which the prime mover is operated until the temperature of the conditioned space is driven down to a selected set point temperature, at which point the prime mover is stopped and the refrigeration unit enters a null cycle, if the battery charging current is below some predetermined value, and the engine coolant temperature is above a predetermined value. When the temperature of the conditioned space rises to some predetermined value relative to the set point temperature, the prime mover is started to again drive the temperature of the conditioned space down to the set point temperature.
If the ambient temperature is close to the selected set point temperature, the null cycle can last indefinitely, at least until the battery voltage drops to some predetermined value, or the temperature of the engine coolant drops to a predetermined value, with either of these events starting the prime mover, even though the temperature of the conditioned space is within the upper and lower temperature limits of the null temperature zone.
Some loads to be conditioned in the conditioned space require continuous air flow, some require at least periodic air flow, and others do not require any minimum amount of air flow. It would be desirable, and it is an object of the present invention, to give the operator of a refrigeration unit more control of air flow during the cycle operating mode, so the air flow may be more closely matched to the requirements of a specific load.
The minimum voltage and charging current values are compromise values selected to accommodate the average battery over its useful operating life, over the complete range of ambient temperature conditions which may be encountered during use of the refrigerant unit. It would be desirable, and it is another object of the invention, to give the operator of the refrigeration unit more control over when an engine prime mover will be automatically started because of the battery voltage, and when an engine prime mover will be prevented from stopping because of the battery charging current.